Smart Grouping control method for power converter switching noise management

ABSTRACT

A switching power converter is provided that cycles a power switch during a group pulse mode of operation to produce a train of pulses within a group period responsive to a control voltage being within a group mode control voltage range. Depending upon the control voltage, the number of pulses in each train of pulses is varied to provide a linear power delivery to the load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/287,377, filed Jan. 26, 2016.

TECHNICAL FIELD

This invention is in the field of power converters, and moreparticularly to circuits and techniques for altering noise emitted byswitching power converters by using a group pulse control method.

BACKGROUND

A flyback switching power converter is typically provided with a mobiledevice as its transformer provides safe isolation from AC current. Likeall switching power converters, a flyback includes a power switchtransistor that is controlled by a controller to regulate the powerdelivery to the load. The cycling of the power switch creates switchingnoise that can affect the mobile device. A mobile device often includesa touchscreen that is becoming more and more sensitive to switchingnoise. For example, users can now operate touchscreens on many phonesand tablets even when wearing gloves. The downside to such highsensitivity touchscreens, and other electrical components, is that theoperation of these components can be susceptible to interference fromthe switching noise. For example, sensing where a touchscreen has beentouched involves a touchscreen sensor monitoring certain frequency bandsas described in detail below. If the electromagnetic interference (EMI)produced by a switching power converter is within the frequency bandmonitored by the touchscreen sensor, then performance of the touchscreenmay be undesirably altered. It will be appreciated that the performanceof other electrical components may also be undesirably altered by theEMI produced by switching power converters.

The interference from switching noise from a switching power convertersuch as a flyback is exacerbated because of the various switching modesused to increase efficiency. In particular, it is conventional to cyclethe power switch using pulse width modulation (PWM) during periods ofrelatively heavy load. The duty cycle (pulse width) is reduced as theload is reduced during PWM operation. But as the load continues toreduce, it is more efficient to cycle the power switch using pulsefrequency modulation (PFM). The various switching frequencies usedduring PFM operation (e.g., from 22 KHz to 89 KHz) spreads the switchingnoise across a relatively wide frequency band such that finding asuitably noise-free band for touchscreen operation may be problematic.

Accordingly, there exists a need to control the frequency bands of EMIproduced by switching power converters.

SUMMARY

During pulse width modulation (PWM) operation at a relatively-heavyload, the switching frequency is maintained constant such that theswitching noise is concentrated at the switching frequency and itsharmonics. It is thus relatively straightforward during PMW operation tofind a suitably noise-free band for touchscreen operation (or operationof other noise-sensitive processes). But it is not efficient to extendPWM operation across the full load range (from very light load to veryheavy load) in a switching power converter. It is thus conventional totransition to pulse frequency modulation (PFM) operation as the load isreduced. But such transition to PFM operation then tends to spread theswitching noise across various frequency bands due to the changingswitching frequencies.

To provide switching noise management, a group pulse mode (GPM) isintroduced in which the switching power converter may transition fromPWM operation (or PFM operation across a limited band) during relativelyheavy load operation to the group pulse mode. To control the transitionto group pulse mode, the controller for controlling the cycling of thepower switch transistor is modified to compare the control voltage to athreshold value. In that regard, it is well known in the powerconversion arts for the controller to respond to a control voltage thatis generated from a comparison of a feedback voltage to a referencevoltage. The feedback voltage is obtained from the output voltage. Forexample, in a flyback converter, the feedback voltage may be sensed fromthe auxiliary winding at the transformer reset time. Alternatively, thefeedback voltage may be sensed through an optoisolator or some otherisolating means between the primary and secondary sides of the flybacktransformer.

The controller compares the feedback voltage to the reference voltage togenerate an error signal that may then be compensated such as through aloop filter to form the control voltage. The controller is configured tocompare the control voltage to a threshold value to control thetransition to group pulse mode control. If the control voltage isgreater than the threshold value, the controller continues to operate inPWM (or PFM) mode in a conventional fashion. But if the control voltageis less than the threshold value, the controller transitions to thegroup pulse mode. As suggested by the name, group pulse mode is effectedwhen the power switch transistor is cycled to produce a train or groupof pulses (each pulse corresponding to an on-time period for the powerswitch transistor in each cycle). Depending upon the control voltage,the size of the group for each burst of pulses is changed. For example,the size of the pulse train may be varied from, for example, fifteenpulses down to one pulse depending upon the value of the controlvoltage. Should the control voltage drop below a low threshold voltage,the controller transitions from group pulse mode to another mode ofcontrol such as PFM. Since such a transition to PFM doesn't occur untila relatively light load occurs (corresponding to the control voltageequaling the relatively-small low threshold voltage), the resulting PFMoperation is denoted herein as a “deep” PFM (DPFM) operation. Thecontrol voltage range for group pulse mode control thus ranges from ahigh threshold voltage to the low threshold voltage.

The control voltage range between the high and low threshold voltagesmay be quantized into a number of group modes, such as 15 group modes.The lowest group mode corresponds to the lowest number of pulses withinthe pulse train (e.g., 1) and starts at the low threshold voltage. Asthe control voltage increases, the number of pulses within thesuccessive group mode increases successively. For example, the number ofpulses in the pulse train may increase by one with each increase in thegroup mode. But the power control by changing the number of pulses fromone group mode to another is relatively coarse. The controller is thusconfigured to also change the group period (the period for one pulsetrain) as a function of the control voltage. In this fashion, a linearpower output is obtained as a function of the control voltage across thevarious group modes.

The resulting group pulse mode control is quite advantageous as thecontrol voltage range may correspond to what would have been a switchingfrequency range of from (for example) approximately 5 KHz to 86 KHz ifconventional PFM operation was employed instead. Such conventional PFMoperation is efficient but spreads the switching noise undesirably. Incontrast, the switching noise for group pulse mode control isconcentrated at the switching frequency for PWM operation (e.g., 86 KHz)and its harmonics. The switching power converter thus enjoys theefficiency of conventional PFM operation over a moderate load range yethas a noise spectrum more like PWM operation. In this fashion,noise-sensitive applications such as touchscreen controllers may readilyfind suitably noise-free bands during group pulse mode operation.

These advantageous features may be better appreciated through aconsideration of the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flyback converter including a controllerconfigured for group pulse mode control in accordance with an aspect ofthe disclosure.

FIG. 2 illustrates the output power for the flyback converter of FIG. 1as a function of the control voltage while the controller is configuredfor the group pulse mode control and also while the controller isconfigured for a modified group pulse mode control in which the groupperiod is held constant for each group mode.

FIG. 3A illustrates the PWM mode, the group modes, and the DPFM mode forthe controller of FIG. 1 as a function of the control voltage inaccordance with an aspect of the disclosure.

FIG. 3B illustrates the group period for the group modes of FIG. 3A.

FIG. 3C illustrates the peak current for the operating modes of FIG. 3A.

FIG. 4 illustrates the control loop within the controller of FIG. 1 inaccordance with an aspect of the disclosure.

FIG. 5 is a flowchart for an example method of operation for thecontroller of FIG. 1 in accordance with an aspect of the disclosure.

FIG. 6 illustrates switching modes as a function of the control voltagein which a group pulse mode is implemented between a pulse frequencymode of operation and a deep pulse frequency mode of operation inaccordance with an aspect of the disclosure.

FIG. 7 illustrates switching modes as a function of the control voltagein which two group pulse modes are implemented for different bands ofswitching frequencies in accordance with an aspect of the disclosure.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

Turning now to the drawings, FIG. 1 shows a fly-back converter 100including a controller 105 configured for group pulse mode (GPM)control. Controller 105 controls the cycling of a power switchtransistor (Q4) such as its turn on time, turn off time, and switchingfrequency to control an output voltage V_(OUT) delivered to a load. Arectified input voltage (Vin) from an AC mains drives a magnetizingcurrent through a primary winding of a transformer T1 when controller105 cycles power switch transistor Q4 on. Controller 105 may then cycleoff power switch transistor Q4 when the current conducted through powerswitch transistor Q4 reaches a desired peak current. To determine whenthe desired peak current has been reached, controller 105 monitors avoltage I_(SENSE) obtained across a sense resistor R_(S) coupled betweena source of power switch transistor Q4 (in an NMOS embodiment for Q4)and ground. When I_(SENSE) reaches the desired peak value, controller105 opens power switch transistor Q4.

Controller 105 includes a feedback loop discussed further below thatdetermines the desired peak current responsive to a feedback voltagesensed from the output voltage V_(OUT). For example, controller 105 maymonitor a feedback voltage V_(SENSE) obtained through a voltage dividerformed by a resistor R1 and R2 coupled in series to an auxiliary windingfor transformer T1. When power switch transistor Q4 is cycled off, themagnetic energy stored within transformer T1 through the build-up of themagnetizing current is released through a pulsing high of a secondarycurrent through a secondary winding for transformer T1. The secondarycurrent is rectified by a diode D₁ and charges an output capacitor C₁with the output voltage V_(OUT). The secondary current then ramps downto zero at a time denoted as the transformer reset time. By sampling thefeedback voltage V_(SENSE) at the transformer reset time, controller 105obtains a feedback voltage that is proportional to the output voltage.As discussed further herein, controller 105 generates an error voltagethrough a comparison of the feedback voltage V_(SENSE) to a referencevoltage. In addition, controller 105 generates a control voltage througha filtering of the error voltage. Controller 105 is configured todetermine whether the control voltage lies within a control voltagerange extending from a low threshold voltage to a high threshold voltageto determine whether to cycle the power switch transistor Q4 accordingto the group pulse mode.

The control voltage range between the high and low threshold voltagesmay be quantized by a number of control voltage range steps, eachvoltage range step corresponding to a group mode. For example, in anembodiment in which the control voltage range is divided into 15equal-sized control voltage range steps, there would be 15 correspondinggroup modes, one for each control voltage range step (vc_step). A lowestgroup mode corresponds to the lowest number of pulses within the pulsetrain (e.g., 1) and starts at the low threshold voltage and extendsacross the first voltage range step. As the control voltage increases,the number of pulses within the successive group mode increasessuccessively. For example, the number of pulses in the pulse train mayincrease by one with each increase in the group mode. But the powercontrol by changing the number of pulses from one group mode to anotheris relatively coarse. Controller 105 is thus configured to also changethe group period (the period for one pulse train) as a function of thecontrol voltage as explained further herein. In this fashion, a linearpower output is obtained as a function of the control voltage across thevarious group modes.

The resulting group pulse mode control is quite advantageous as thecontrol voltage range may correspond to what would have been a switchingfrequency range of from (for example) approximately 5 KHz to 86 KHz ifconventional PFM were employed instead. Such conventional PFM operationis efficient but spreads the switching noise undesirably. In contrast,the switching noise for group pulse mode control is concentrated at theswitching frequency for PWM operation (e.g., 86 KHz) and its harmonics.The switching power converter thus enjoys the efficiency of conventionalPFM operation over a moderate load range yet has a noise spectrum morelike PWM operation. In this fashion, noise-sensitive applications suchas touchscreen controllers may readily find suitably noise-free bandsduring group mode control operation.

Some theory for the group mode pulse control will now be discussed. Themathematics is simplified if the low threshold voltage Vth is replacedby a modified low threshold voltage Vth′ that equals (Vth−vc_step). Thenumber of pulses within each pulse train for a given group mode can beexpressed as an integer function of a ratio of voltage range from themodified low voltage threshold to the voltage range step. In particular,the real value of this ratio is as follows:N_group_real(Vc)=(Vc−Vth′)/vc_step

where N_group_real (Vc) is the real value of the ratio, Vc is thecontrol voltage, Vth′ is the modified low threshold voltage, and vc_stepis the voltage range step. Given this real function, the integer numberof the pulses within the pulse train for a given group mode is:N _(group(Vc))=floor×(N _(group) _(real(Vc)) )+1

where N_group(Vc) is the integer number of pulses for the pulse train inthe group mode corresponding to the voltage range step containing thecurrent value of the control voltage (Vc) and floor(X) is a functionthat takes the nearest integer value of its argument.

With the burst group period for each pulse train defined as “Tp_group”the resulting output power for flyback converter 100 as a function ofthe control voltage Vc becomes:

${{P\_ int}({Vc})} = {\frac{1}{2} \times {Lm} \times {Ip}^{2} \times \left( \frac{{N\_ group}({Vc})}{Tp\_ group} \right) \times \eta}$

where Lm is the magnetizing inductance of the primary winding, Ip is thepeak primary current value, and η is the power conversion efficiency. Ifthe burst group period Tp_group was constant across all the group modes,the output power of flyback converter 100 as a function of the controlvoltage would be as shown for curve 200 in FIG. 2. The quantization ofoutput power by increasing the number of pulses by one from one groupmode results in an undesirably stepping of the output power. To make theoutput power a linear function of the control voltage, the group periodmay be varied as a function of the control voltage as follows:Tp_group(Vc)=Tp_group0×(N_group(Vc)/N_group_real(Vc))

where Tp_group(Vc) is the group period as a function of the controlvoltage, and Tp_group0 is a constant group period value. The groupperiod at the lowest value of the control voltage for a given group modeover a corresponding voltage range step will thus begin at Tp_group0 andthen decrease as the control voltage increases. It can then be shownthat the output power as a function of the control voltage (P(Vc)) is:

${P({Vc})} = {\frac{1}{2} \times {Lm} \times {Ip}^{2} \times \left( \frac{{N\_ group}({Vc})}{{Tp\_ group}({Vc})} \right) \times \eta}$

where P(Vc) is represented by line 205 in FIG. 2 and is a smooth linearfunction of the control voltage. The group period is thus varied toaccount for the discontinuous steps in the original power function 200.

An example division of the control voltage range is shown in FIG. 3A.Controller 105 is configured to operate in PWM mode when the controlvoltage Vc is higher than the high threshold voltage (Vhigh). During PWMoperation, the switching frequency is relatively high such as 86 KHz.The switching noise during PWM operation will thus be concentrated at 86KHz and its harmonics. This same switching frequency is used for thepulse trains in the group modes when the control voltage is in the groupmode control voltage range ranging from the low threshold voltage (Vth)to the high threshold voltage Vhigh. In the embodiment shown in FIG. 3A,the control voltage step for each group mode is such that the group modecontrol range is divided into fifteen control voltage steps. A firstcontrol voltage step starting at the low threshold voltage Vthcorresponds to a group mode 1. The group modes range from group mode 1to a group mode 15. Each successive group mode has one more pulse in itspulse train as compared to the preceding group mode. The pulse train forgroup mode 1 is thus one pulse long. The pulse train for group mode 2would be two pulses long, the pulse train for group mode 3 would bethree pulses long, and so on such that a final group mode 15 has a pulsetrain that is fifteen pulses long. It will be appreciated that thenumber of initial pulses in group mode 1 may be greater than one inalternative embodiments.

The switching frequency for the pulses in each pulse train may match theswitching frequency used for the PWM operation (e.g., 86 KHz). When thecontrol voltage drops below the low threshold voltage Vth, controller105 begins a deep PFM (DPFM) mode of operation that may range from astarting pulse frequency such as 5.4 KHz. The pulse frequency for DPFMoperation drops toward zero as the control voltage is reduced furtherfrom the low threshold voltage Vth. The starting pulse frequency for thetransition into the DPFM mode equals a reciprocal of the default groupperiod Tp_group0.

Referring again to GPM operation, the pulse train length or number ofpulses within each pulse train (N_group) for each group mode alsocorresponds to the group mode number. For example, group mode 15 has apulse train length of fifteen, which is the maximum value (Nmax) forN_group. It may thus be shown that the high threshold voltage Vhigh=lowthreshold voltage Vth+Nmax*vc_step.

The group period Tp_group as a function of the control voltage for GPMoperation is shown in FIG. 3B for the group modes of FIG. 3A. The groupperiod begins at Tp_group0 at the start (lowest value of the controlvoltage) for each group mode and then decreases. The difference betweenthe starting (default) value Tp_group0 and the final value of the groupperiod (the highest value of the control voltage) for each group mode isgreatest for the lowest group mode (group mode 1) and smallest for thehighest group mode (group mode 15). The peak current (Ip) for the DPFMoperation, group mode operation, and PWM operation of FIG. 3A is shownin FIG. 3C. The peak current for each pulse is a constant value for boththe DPFM and group mode operation whereas the peak current increaseslinearly as a function of the control voltage during PWM operation.

The control loop for controller 105 is shown in more detail in FIG. 4.The output voltage V_(OUT) is sampled as discussed with regard toflyback converter 100 through a transformer and sensing network 405 thatrepresents the auxiliary winding of the transformer and the voltagedivider of FIG. 1. Controller 105 includes a sensor 410 such as ananalog-to-digital converter (ADC) that converts the feedback voltageinto a digitized version vfb_dig. A comparator 415 compares thedigitized feedback voltage vfb_dig to a digital reference voltage vrefto produce a digital error signal Verr. It will be appreciated thatcomparator 415 may be replaced by an error amplifier in an analogversion for controller 105 in alternative embodiments. The error signalVerr is processed through a loop filter 420 to produce the controlvoltage 420. Regardless of whether the processing of the feedbackvoltage is digital or analog, the control voltage is an analog voltageas discussed with regard to FIG. 3A. A loop control circuit 425determines the pulse train number N_group(Vc) and the group periodTp_group(Vc) as discussed above. Controller 105 then cycles the powerswitch transistor 105 with the pulse train number of pulses within thegroup period as represented by a power conversion process 430. Dependingupon the electrical properties of the transformer T1 and the outputstage as represented by output filter 435, the flyback converterproduces the output voltage V_(OUT).

A flowchart for a method of operation for controller 105 is shown inFIG. 5. The method begins with the control voltage being greater thanthe high threshold voltage Vhigh, which equals the sum of the lowthreshold voltage Vth and (Nmax)*vc_step as discussed earlier such thatcontroller operates in the PWM mode during an initial step 500. The ontime (Ton) in each cycle of the power switch transistor Q4 is given as aproduct of a default on time Ton0 with a pulse width control function(fpwm) of the control voltage Vc. Each power switch cycle has a fixedperiod of Tp0. Controller 105 determines whether the control voltage hasdropped below the high threshold voltage Vhigh in a step 505. Note thatdetermining whether the control voltage is less than the high thresholdvoltage Vhigh is equivalent to determining whether the control voltageis less than a sum of the modified low threshold voltage Vth′ with theproduct of (Nmax+1) and vc_step. If the determination in step 505 isnegative, then operation continues in the PWM mode (step 400). If thedetermination in step 505 is positive, then group mode control begins inan step 510.

During group mode control, the switching frequency for each cycle of thepulse train is given by the fixed period Tp0 used during PWM operation.The on time of each pulse within the pulse train is the default valueTon0. Based upon the control voltage, controller 104 determines thepulse train number N_group (the group mode) and also the group periodTp_group.

Should the control voltage drop below the low threshold voltage Vth asdetermined in a step 515, controller begins DPFM operation in a step520. The period for the switching frequency during DPFM operation isgiven by a product of Tp_group0 and a DPFM function (fdpwm) of thecontrol voltage. The on time within each period is the default on timeTon0.

The table below lists parameters used by an embodiment of controller 105for the PWM, group mode control, and DPFM modes of operation, where Fmaxis the switching frequency for the PWM mode of operation.

Mode Vc N T_(on) T_(p) PWM Vc >= V_(th)′ + N_(max) + 1 T_(on()) *1/F_(max) (N_(max) + 1) * V_(step) f_(pwm)(Vc) Group Vc < V_(th)′ +Floor T_(on()) (N_(max) + 1)/F_(max) * (N_(max) + 1) * V_(step) ((Vc −N * V_(step)/ Vc >= (Vth′ + V_(step)) V_(th)′)/V_(step)) (Vc − V_(th)′)DPFM Vc < (Vth′ + V_(step)) 1 T_(on()) (N_(max) + 1)/F_(max) *f_(dpfm)(Vc)

Rather than transition to the group mode control from a PWM mode ofoperation, the transition can instead be from a PFM mode of operation asshown in FIG. 6. In such an embodiment, controller 105 may operate usingthe following parameters:

Mode Vc N T_(on) T_(p) PWM F_(max) > F_(limit*) * f₂(Vc) N_(max)T_(on()) * 1/F_(limit) f₁(Vc) PFM F_(max) < F_(limit*) * f₂(Vc) N_(max)T_(on()) (N_(max)+ 1)/F_(max) * Vc >= V_(th)′ + f₂(Vc) (N_(max) +1)*V_(step) Group Vc <= V_(th)′ + Floor T_(on()) (N_(max) + 1/F_(max) *(N_(max) + 1) * V_(step) ((Vc − N * V_(step)/ Vc >= V_(th)′)/V_(step))(Vc − V_(th)′) (V_(th)′ + vc_step) DPFM Vc < (V_(th)′ + vc_step) 1T_(on()) N_(max)/F_(max) * f_(dpfm)(Vc)

In yet another alternative embodiment, controller 105 may be configuredto implement multiple group pulse modes as shown in FIG. 7. It will befurther appreciated that the GPM modes may be selected around certainfrequency bands such as the frequency bands A and B of FIG. 7 to provideEMI management around those frequency bands as described herein.

Those of some skill in this art will by now appreciate and depending onthe particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the scope thereof. In light of this,the scope of the present disclosure should not be limited to that of theparticular embodiments illustrated and described herein, as they aremerely by way of some examples thereof, but rather, should be fullycommensurate with that of the claims appended hereafter and theirfunctional equivalents.

We claim:
 1. A switching power converter controller, comprising: a loopfilter for filtering an error signal to produce a control voltage; aloop control logic circuit configured to quantize a first function of adifference between the control voltage and a threshold voltage todetermine an integer number of pulses for each pulse in a pulse trainresponsive to the control voltage being within a group mode controlvoltage range for a group pulse mode of operation and to determine agroup period for the pulse train that is proportional to a product of adefault group period and a second function of the difference, whereinthe loop control logic circuit is further configured to command a powerswitch to cycle according to the integer number of pulses within thegroup period responsive to the control voltage being within the groupmode control voltage range, wherein the group mode control voltage rangeis divided into a plurality of equal control voltage steps, and whereinthe first function of the difference equals a ratio of the difference tothe equal control voltage step, and wherein the loop control logiccircuit is configured to quantize the first function by taking a nearestinteger value of the first function.
 2. The switching power convertercontroller of claim 1, wherein the loop control logic circuit is furtherconfigured to command the power switch to cycle according to a pulsewidth modulation mode of operation responsive to the control voltagebeing greater than an upper limit for the group mode control voltagerange.
 3. The switching power converter controller of claim 2, whereineach cycle of the power switch during the pulse width modulation mode ofoperation occurs according to a pulse width modulation (PWM) switchingfrequency, and wherein the loop control logic circuit is furtherconfigured to command the power switch to cycle according to the integernumber of pulses at the PWM switching frequency.
 4. The switching powerconverter controller of claim 1, wherein the loop control logic circuitis further configured to cycle the power switch according to a pulsefrequency mode of operation responsive to the control voltage beinggreater than an upper limit of the group mode control voltage range. 5.The switching power converter controller of claim 1, wherein the loopcontrol logic circuit is further configured to cycle the power switchaccording to a deep pulse frequency mode of operation responsive to thecontrol voltage being less than a lower limit for the group mode controlvoltage range.
 6. The switching power converter controller of claim 1,further comprising: a sensor configured to sense and digitize a feedbackvoltage to form a digital feedback voltage; and a comparator configuredto compare the digital feedback voltage to a digital reference voltageto form the error signal.
 7. The switching power converter controller ofclaim 1, wherein the second function equals a product of the defaultgroup period with a ratio of the integer number of pulses to the firstfunction of the difference.
 8. The switching power converter controllerof claim 1, wherein the switching power converter controller is aprimary-side controller for a flyback converter.
 9. A method,comprising: processing a feedback voltage for a switching powerconverter to provide a control voltage; and responsive to the controlvoltage being within a group mode control voltage range: determining avalue for a first function of a difference between the control voltageand a threshold voltage; quantizing the value of the first function todetermine an integer number of pulses for a pulse train; adjusting adefault group period responsive to a second function of the differenceto provide an adjusted group period; and cycling a power switchaccording to the integer number of pulses within the adjusted groupperiod, wherein the first function of the difference equals a ratio ofthe difference to an increment of the group mode control voltage range,and wherein quantizing the value of the first function comprises takinga nearest integer value of the value of the first function.
 10. Themethod of claim 9, further comprising: cycling the power switchaccording to a pulse width modulation mode of operation responsive tothe control voltage being greater than an upper limit of the group modecontrol voltage range.
 11. The method of claim 10, wherein cycling thepower switch according to the pulse width modulation mode of operationoccurs according to a pulse width modulation (PWM) switching frequency,and cycling of the power switch according to the integer number ofpulses occurs at the PWM switching frequency.
 12. The method of claim10, further comprising: cycling the power switch according to a pulsefrequency modulation mode of operation responsive to the control voltagebeing greater than an upper limit of the group mode control voltagerange.
 13. The method of claim 9, wherein the second function equals aproduct of the default group period with a ratio of the integer numberof pulses and the first function of the difference.
 14. The method ofclaim 9, wherein the power switch is a power switch for a flybackconverter.
 15. The method of claim 14, further comprising: sensing anauxiliary winding voltage through a voltage divider to form the feedbackvoltage.
 16. The method of claim 15, wherein processing the feedbackvoltage comprises comparing a digital version of the feedback voltagewith a digital reference voltage to form a digital error signal andfiltering the digital error signal to form the control voltage.
 17. Themethod of claim 9, further comprising: cycling the power switchaccording to a deep pulse frequency mode of operation responsive to thecontrol voltage being less than a lower limit for the group mode controlvoltage range.